What you can do is save as much water as you can, and mix it with the rest of new water. If you save 50% of old water and add 50% of new water, maybe you could get away with acclimatizing (if your old water has
pH and
nitrate similar to your tap water). But if your tap nitrate and pH are significantly different from your tank nitrate and pH, I'd recommend acclimatization even if you save 50% of water. If, however, you save more than 50% (say, 70 - 80%) and your tap pH and nitrate are different from tank pH and nitrate, you would not need acclimatization in such a case, because it would be more like a
water change.